\begin{tikzpicture}[>=Stealth]
    \draw [->] (-1,0) -- (4,0) node[anchor=north] {$n$};
    \draw [->] (0,-1) -- (0,5.5) node[anchor=east] {$a_n$};
    \node at (-0.2,-0.2) {$O$};
    \foreach \x in {1,...,7} {
        \draw (\x/2,0.2) -- (\x/2,0) node[anchor=north] {$\x$};
    }
    \draw (0.2, 5) -- (0,5) node[anchor=east] {$1$};
    \foreach \y in {2, 4, 8} {
        \draw (0.2, 5/\y) -- (0,5/\y) node[anchor=east] {$\frac{1}{\y}$};
    }

    \foreach \n in {1,...,7} {
        \filldraw [fill=black] (\n/2, 5/\n) circle (0.1);
    }
\end{tikzpicture}

